This application relates efficiency-driven operation of dispatchable sources and storage units in energy systems.
Distributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from many small energy sources. Most countries generate electricity in large centralized facilities, such as fossil fuel (coal, gas powered), nuclear, large solar power plants or hydropower plants. These plants have excellent economies of scale, but usually transmit electricity long distances and negatively affect the environment. Distributed generation allows collection of energy from many sources and may give lower environmental impacts and improved security of supply.
Right-sized resources such as microgrids are able to offer important but little-known economic advantages over central plants. Smaller units offered greater economies from mass-production than big ones could gain through unit size. Batteries can act as a buffer to alleviate the mismatch of generation and demand in a microgrid. In this way, when DGs output power is more than the demand, battery is charged. The battery is discharged during times of low generation and high demand to reduce the power mismatch.
Due to rapid changes in the power output of renewable energy sources over time and variations in the demand, a battery might experience a very irregular pattern of charge and discharge in a microgrid if not controlled properly. This will have a negative impact on battery lifetime and will increase the overall operational cost of the microgrid. Therefore, in addition to balancing supply and demand in real-time, power management system should operate the battery in a way to minimize operational cost of a microgrid.
Since one goal of controller in energy usage optimization of microgrids is to reduce the cost of consumed energy for the end-users, it is necessary to calculate or obtain the unit price of energy from each generator, energy storage unit, and the grid (in case of grid-tied microgrids) at each step of microgrid operation. In this way the controller can identify the cheapest sources of energy in a microgrid and send commands to them in order to match the electricity supply and demand in the system.
Conventional systems are based on passive control of energy storage units. Examples include peak-shaving control were a storage unit discharges only if the load exceeds a certain threshold. Another example is schedule-based control in which a storage unit charges and discharges at certain times during the day. However, passive control lacks the general knowledge about real-time changes in generation and demand levels as well as operational costs of the system; thus it cannot guarantee an optimal operation of the storage unit.
In energy systems with distributed generations and energy storage units (Distributed energy resources, DERs), the demand at each instant is supplied through a combination of different sources including discharging the energy storage unit. Since each DER unit has a unique efficiency versus input/output characteristic, the overall supply of load in the system might happen at low efficiencies during certain periods of operation. Low efficiency operation increases per unit cost of energy delivered to the load due to the excessive losses in the system. Previous attempts were based on improving the quality of design and materials used in different DER units to increase the efficiency of individual generation resources in the system. Other attempts include sizing the units in a way to ensure high-efficiency operation all the time. Mechanical systems such as gearboxes are also used in conjunction with rotating generators to increase the efficiency of generation in the system.